The insulin-like growth factor 1 receptor called IGF-1R (or sometimes IGF1R or IGF-IR) is a receptor with tyrosine kinase activity having 70% homology with the insulin receptor IR. IGF-1R is a glycoprotein of molecular weight approximately 350,000. It is a hetero-tetrameric receptor of which each half-linked by disulfide bridges—is composed of an extracellular α-subunit and of a transmembrane β-subunit. IGF-1R binds IGF1 and IGF2 with a very high affinity (Kd #1 nM) but is equally capable of binding to insulin with an affinity 100 to 1000 times lower. Conversely, the IR binds insulin with a very high affinity although the IGFs only bind to the insulin receptor with a 100 times lower affinity. The tyrosine kinase domain of IGF-1R and of IR has a very high sequence homology although the zones of weaker homology respectively concern the cysteine-rich region situated on the α-subunit and the C-terminal part of the β-subunit. The sequence differences observed in the α-subunit are situated in the binding zone of the ligands and are therefore at the origin of the relative affinities of IGF-1R and of IR for the IGFs and insulin respectively. The differences in the C-terminal part of the β-subunit result in a divergence in the signalling pathways of the two receptors; IGF-1R mediating mitogenic, differentiation and antiapoptosis effects, while the activation of the IR principally involves effects at the level of the metabolic pathways.
The cytoplasmic tyrosine kinase proteins are activated by the binding of the ligand to the extracellular domain of the receptor. The activation of the kinases in turn involves the stimulation of different intra-cellular substrates, including IRS-1, IRS-2, Shc and Grb 10. The two major substrates of IGF-1R are IRS and Shc which mediate, by the activation of numerous effectors downstream, the majority of growth and differentiation effects connected with the attachment of the IGFs to this receptor. The availability of substrates can consequently dictate the final biological effect connected with the activation of the IGF-1R. When IRS-1 predominates, the cells tend to proliferate and to transform. When Shc dominates, the cells tend to differentiate. It seems that the route principally involved for the effects of protection against apoptosis is the phosphatidyl-inositol 3-kinases (PI 3-kinases) route.
The role of the IGF system in carcinogenesis has become the subject of intensive research in the last ten years. This interest followed the discovery of the fact that in addition to its mitogenic and antiapoptosis properties, IGF-1R seems to be required for the establishment and the maintenance of a transformed phenotype. In fact, it has been well established that an overexpression or a constitutive activation of IGF-1R leads, in a great variety of cells, to a growth of the cells independent of the support in media devoid of foetal calf serum, and to the formation of tumors in nude mice. This in itself is not a unique property since a great variety of products of overexpressed genes can transform cells, including a good number of receptors of growth factors. However, the crucial discovery which has clearly demonstrated the major role played by IGF-1R in the transformation has been the demonstration that the IGR-1R″ cells, in which the gene coding for IGF-1R has been inactivated, are totally refractory to transformation by different agents which are usually capable of transforming cells, such as the E5 protein of bovine papilloma virus, an overexpression of EGFR or PDGFR, the T antigen of SV40, activated ras or the combination of these two last factors.
IGF-1R is expressed in a great variety of tumors and of tumor lines and the IGFs amplify the tumor growth via their attachment to IGF-1R. Other arguments in favor of the role of IGF-1R in carcinogenesis come from studies using murine monoclonal antibodies directed against the receptor or using negative dominants of IGF-1R. Actually, murine monoclonal antibodies directed against IGF-1R inhibit the proliferation of numerous cell lines in culture and the growth of tumor cells in vivo. It has likewise been shown that a negative dominant of IGF-1R is capable of inhibiting tumor proliferation.
A large number of projects have been initiated to develop naked IGF-1R antibodies for the treatment of cancers. Nevertheless, at this date, none of these projects have been successful and there are no anti-IGF-1R antibodies on the market.
Moreover, a series of clinical trials involving anti-IGF-1R antibodies combined to anti-EGFR antibodies in order to target both EGFR and IGF-1R, have failed as none of these antibodies were able to treat KRAS mutant patients.
As a consequence, IGF-1R is not considered now as a major target and, in the research of potential therapeutic antibodies, IGF-1R is no more considered as of particular interest.
Nevertheless, it must also be noticed that endeavours to generate IGF-1R antibodies were focussed on naked antibodies, i.e. antibodies useful by their intrinsic properties. In this sense, IGF-1R is considered as a target not suitable for the generation of an ADC such as an antibody-drug-conjugate (referred as “ADC”) as IGF-1R is described as a target also widely expressed by normal cells, including blood vessels. In this sense, it can be noticed that the most recent IGF-1R antibody, i.e. AVE1642, is developed as a naked antibody not armed with a drug. It is the same with the other IGF-1R antibodies currently in development and with all those which failed in clinical trials.
In this context, the invention relates to an ADC or conjugate and its use for the treatment of cancer, and more particularly IGF-1R-expressing cancers.
ADCs combine the binding specificity of an antibody with the potency of drugs such as, for example, cytotoxic agents. The technology associated with the development of monoclonal antibodies, the use of more effective drugs and the design of chemical linkers to covalently bind these components, has progressed rapidly in recent years.
The use of ADCs allows the local delivery of drugs which, if administered as unconjugated drugs, may result in unacceptable levels of toxicity to normal cells.
In other words, maximal efficacy with minimal toxicity is sought thereby. Efforts to design and refine ADC have focused on the selectivity of antibody as well as drug mechanism of action, drug-linking, drug/antibody ratio (loading or DAR), and drug-releasing properties. Drug moieties may impart their cytotoxic and cytostatic effects by mechanisms including tubulin binding, DNA binding, proteasome, impairment of ribosome function, protein synthesis and/or topoisomerase inhibition. Some cytotoxic drugs tend to be inactive or less active when conjugated to large antibody.
Each antibody must be characterized separately, an appropriate linker designed, and a suitable cytotoxic agent identified that retains its potency upon delivery to tumor cells. One must consider the antigen density on the cancer target and whether normal tissues express the target antigen. Other considerations include whether the entire ADC is internalized upon binding the target; whether a cytostatic or cytotoxic drug is preferable when considering possible normal tissue exposure and/or the type and stage of the cancer being treated; and, whether the linker connecting the antibody to the drug payload is a cleavable or a non-cleavable linkage. Furthermore, the antibody to drug moiety conjugation ratio must be sufficient without compromising the binding activity of the antibody and/or the potency of the drug and without modifying physicochemical properties of the ADC resulting on aggregation or deleterious properties regarding to the future development process of the compound.
An ADC is a complex biological molecule and the challenges to develop an effective ADC remain a significant issue.